Interferometric sensing apparatus including adjustable coupling and associated methods

ABSTRACT

A sensing apparatus comprises an excitation source configured to induce waves in a target, and a fiber optic interferometer configured to sense the induced waves in the target. The fiber optic interferometer comprises a probe segment having a probe segment end, and an adjustable coupler configured to permit setting a gap between the probe segment end and the target. A controller is coupled to the adjustable coupler and configured to set the gap between the probe segment end and the target.

GOVERNMENT CONTRACT

This invention was made with Government support under GovernmentContract 03-180 awarded by the FBI. The Government has certain rights inthis invention.

FIELD OF THE INVENTION

The present invention relates to the field of interferometers and, moreparticularly, to fiber optic interferometers and related methods.

BACKGROUND OF THE INVENTION

Ultrasonic waves may be used to probe a variety of materials,particularly for thickness gauging and flaw detection. The ultrasonicwaves are typically generated with a piezoelectric transducer. Theultrasonic waves propagate through the material, reflecting frominterfaces (in thickness gauging applications) or internal features (inflaw detection applications). The scattered ultrasonic waves propagateback to the surface of the material, causing the surface to vibrate atthe ultrasound frequency. This vibration may be detected with apiezoelectric transducer similar to the one used to generate theultrasonic waves, and then analyzed to generate data about the material.

Optical detection techniques can be used in place of the piezoelectrictransducers to remotely detect the ultrasonic waves. Generally, a laserprobe beam is directed onto the material. When the surface vibrates itimparts a phase shift onto the reflected beam. This phase shift isdetected with a photodetector after mixing the reflected probe beam witha stable reference beam and measuring the amplitude and frequency orphase of the photodetector output intensity fluctuations. The referencebeam originates from the same laser source as the reflected probe beam,and the output signal from the photodetector corresponds to the surfacemotion.

One problem with laser detection systems is low sensitivity. Typically,the material surface that is being probed has a diffusely reflecting orscattering quality. Consequently, the reflected beam is highly aberratedand its wavefront is mismatched with respect to the reference beam. Theresulting signal produced by the photodetector is therefore weak andlacks precision.

In U.S. Pat. No. 6,075,603 to O'Meara, a contactless system for imagingan acoustic source within a workpiece is disclosed. In this system, anarray of discrete optical detectors are arranged in a pattern. A probebeam is directed onto a vibrating surface in a pattern that correspondsto the detector array. The probe beam is reflected onto the detectorarray and a reference beam is also directed onto the detector array atan angle to the probe beam to produce fringe patterns on the detectorsthat correspond to the surface vibration pattern. A readout systemutilizes the discrete detector outputs to produce an array output signalindicative of at least a size and two dimensional location for theacoustic source relative to the vibrating surface. This system, however,may not provide the desired accuracy, and may be sensitive tofluctuations in the length of the paths between the probe beam and thesurface, and the reference beam and the surface.

U.S. Pat. No. 7,262,861 to Pepper discloses a laser ultrasonicinspection apparatus which enables remote sensing of thickness,hardness, temperature and/or internal defect detection. A lasergenerator impinges a workpiece with light for generating athermo-elastic acoustic reaction in a workpiece. A probe laser impingesthe workpiece with an annularly-shaped probe light for interaction withthe acoustic signal in the workpiece resulting in a modulated returnbeam. A photodetector having a sensitive region is for detecting anannularly-shaped fringe pattern generated by an interaction of areference signal with the modulated return beam at the sensitive region.

This system, however, may not provide the desired accuracy, and may besensitive to fluctuations in the length of the path between the probebeam and the surface, or fluctuations in the path lengths of thereference and measurement arms of the interferometer.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a sensing apparatus. This and otherobjects, features, and advantages in accordance with the presentinvention are provided by a sensing apparatus permitting adjustment of agap between an interferometer probe and a target that includes anexcitation source configured to induce waves in a target, and a fiberoptic interferometer configured to sense the induced waves in thetarget. The fiber optic interferometer comprises a probe segment havinga probe segment end, and an adjustable coupler configured to permitsetting a gap between the probe segment end and the target. A controlleris coupled to the adjustable coupler and configured to set the gapbetween the probe segment and the target.

The fiber optic interferometer may also include a laser source, and aphotodetector coupled to the controller. A optical coupler operativelyconnects the laser source, the photodetector, and the probe segment. Thecontroller is further configured to generate target data based upon thesensed induced waves.

Setting the gap between the probe segment and the target advantageouslyallows the fiber optic interferometer to be tuned such that the distancebetween the probe segment end and the target is a desired fraction ormultiple of the wavelength of light emitted from the probe segment end.This helps to minimize distortions in the generated target data.

In some applications, the adjustable coupler may comprise apiezoelectric body. Alternatively, the adjustable coupler may comprise asleeve surrounding the probe segment end and at least one of a heatingsource and a cooling source associated therewith. In some applications,the heating source may be a laser.

The adjustable coupler comprises a sleeve surrounding the probe segmentend. In addition, the adjustable coupler may further comprise a biasingmember configured to urge the sleeve in contact with the target. Theprobe end comprises an optical fiber with an angled endface, and theexcitation source may comprise at least one pulsed laser.

A method aspect is directed to a method of sensing a target comprisinginducing waves in the target using an excitation source, and sensing theinduced waves in the target using a fiber optic interferometercomprising a probe segment end by at least setting a gap between theprobe segment end and the target.

According to another aspect, a sensing apparatus comprises an excitationsource configured to induce waves in a target, and a fiber opticinterferometer configured to sense the induced waves in the target. Thefiber optic interferometer comprises a laser source, and a probe segmenthaving a probe segment end to be positioned adjacent the target anddefining a gap therebetween. A optical coupler is operatively connectingthe laser source and the probe segment. In addition, a controllercoupled is to the laser source and configured to adjust a wavelength ofthe laser source so that a desired multiple of the wavelength equals thegap between the probe segment end and the target.

According to a further aspect, a sensing apparatus includes anexcitation source configured to induce waves in a target, and a fiberoptic interferometer configured to sense the induced waves in thetarget. The fiber optic interferometer comprises a laser source, and aprobe segment having a probe segment end to be positioned adjacent thetarget and defining a gap therebetween. An optical coupler operativelyconnects the laser source and the probe segment, and a controller iscoupled to the laser source and configured to adjust a wavelength of thelaser source so that a desired multiple of the wavelength equals the gapbetween the probe segment end and the target.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a sensing apparatus, according tothe present invention.

FIG. 2 is a schematic sectional view of an adjustable coupler, as usedwith the sensing apparatus of FIG. 1.

FIG. 3 is a schematic sectional view of another adjustable coupler, suchas may be used with the sensing apparatus of FIG. 1.

FIG. 4 is a schematic sectional view of yet another adjustable coupler,such as may be used with the sensing apparatus of FIG. 1.

FIG. 5 is a schematic cross sectional view of an additional adjustablecoupler, such as may be used with the sensing apparatus of FIG. 1.

FIG. 6 is a schematic block diagram of another embodiment of a sensingapparatus, according to the present invention.

FIG. 7 is a flowchart of a method of sensing a target in accordance withthe present invention.

FIG. 8 is a partial schematic sectional view of an adjustable coupler ofa biological sensing apparatus sensing an arterial wall in accordancewith the present invention.

FIG. 9 is a partial schematic sectional view of an adjustable coupler ofa material inspection apparatus sensing a weld in accordance with thepresent invention.

FIG. 10 is a schematic block diagram of another sensing apparatus inaccordance with the present invention.

FIG. 11 is a schematic block diagram of yet another sensing apparatus inaccordance with the present invention.

FIG. 12 is a flowchart of another method of sensing a target inaccordance with the present invention.

FIG. 13 is a partial schematic sectional view of a biological sensingapparatus sensing an arterial wall in accordance with the presentinvention.

FIG. 14 is a partial schematic sectional view of a material inspectionapparatus sensing a weld in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime and multiple primenotations are used to indicate similar elements in alternativeembodiments.

Referring initially to FIG. 1, a sensing apparatus 10 in accordance withthe present invention is now described. The sensing apparatus 10 is usedto sense, or determine, a variety of properties of a target 12,including, for example, the dimensions of the target, the materialcomposition of the target, and the thickness of the target.

The sensing apparatus 10 includes an excitation source 14,illustratively a pulsed laser, configured to induce ultrasonic waves inthe target 12. The excitation source 14 is an optical source and isillustratively coupled to the target 12 via an optical fiber 15 havingan end portion 16 in physical contact with the target, although itshould be appreciated that in some embodiments the excitation source isnot coupled to the target via an optical fiber but rather radiates thetarget via free space. The excitation source 14 induces the ultrasonicwaves in the target 12 by rapidly heating it. It should be appreciatedthat in some applications, the excitation source 14 may be a broadbandoptical source, or a doped fiber amplifier.

A fiber optic interferometer 17 senses the induced waves and generatestarget data based thereupon. In particular, the fiber opticinterferometer 17 comprises a probe segment 30 having a probe segmentend 31 coupled to the target 12. The interferometer laser source 18 isconnected to an adjustable coupler 14 via optical fibers 21 and throughan optical isolator 19 and an optical coupler 22. Also coupled to theoptical coupler 22 is a photo-detector 20 via optical fiber 23.

The adjustable coupler 24 is in physical contact with the target 12, andpermits setting a gap between the probe segment end 31 and the target. Acontroller 26 is coupled to the adjustable coupler 24 and is configuredto control the adjustable coupler to thereby set the gap between theprobe segment end 31 and the target 12.

Operation of the fiber optic interferometer 17 is now described. Theinterferometer laser source 18 radiates the target 12 via the probesegment end 31. A portion of the light radiating within the probesegment 30 is reflected back as it hits the probe segment end 31,through the optical coupler 22, and into the photodetector 20.Similarly, a portion of the light radiating within the probe segment 30is radiated from the probe segment end 31 onto the target 12. This lightis then reflected from the target 12 back into the probe segment 30 viathe probe segment end 31, through the optical coupler 22, and into thephotodetector 20. Consequently, the light reflected from the probesegment end 31 and the light reflected from the target 12 will combine,and the superposition thereof is detected by the photodetector 20. Thelight reflected by the target will typically undergo a phase change dueto the ultrasonic waves and resulting vibrations in the target 12, andtherefore will have a different phase than the light reflected by theprobe segment end 31, causing constructive and destructive interferenceto occur therebetween. This interference therefore reflects a detectionof the sensed induced waves and can be analyzed in order to determinevarious properties of the target, as will be appreciated by thoseskilled in the art.

The controller 26 generates target data based upon the sensed inducedwaves. To do so, a laser pulse from the pulsed laser 14 triggers thestart of a measurement cycle, performed by the controller 26, in thetime domain. Signal peaks observed by the controller 26 correspond tothe transmit time of surface waves from the point of excitation (thatis, the point of the target 12 on which the pulse from the pulsed laser14 radiates) to the probe segment end 31. Since the distance between theexcitation point and the probe segment end 31 is known, the acousticvelocity of the ultrasonic waves in the target 12 can be calculated. Bycomparing this acoustic velocity to a table of acoustic velocity fordifferent materials, the material composition of the target can bedetermined. It should be understood that the adjustable coupler 24 andexcitation source probe 16 can be scanned to different locations on thetarget 12, so as to gather information about many points of the target.

The controller 26 may include a processor and a memory cooperatingtherewith. The memory may be volatile or non-volatile, and the processormay be an integrated circuit, in some applications.

In addition, the controller 26 performs typical interferometriccalculations as known to those of skill in the art on the superpositionof the light reflected by the probe segment end 31 and the lightreflected by the target 12 to potentially determine the dimensionsand/or the thickness of the target. Since a difference in the length ofthe path traveled by the light reflected by the probe segment end 31 andthe light reflected by the target 12 will result in an additional phasedifference therebetween, it is desirable for the difference in thelength of that path to remain the same. That is, it desirable for thegap between the probe segment end 31 and the target 12 to remainconstant, such that the gap is a desired multiple of the wavelength ofthe light radiated by, and reflected into, the probe segment end 31′.The multiple used need not be an integer in some embodiments, and neednot be greater than one in some embodiments.

As stated above, the controller 26 controls the adjustable coupler 24 toadjust the gap. As shown in FIG. 2, the adjustable coupler 24, in someembodiments, may comprise a sleeve 29 surrounding probe segment end 31,and a biasing member 28 to urge the sleeve 29 in physical contact withthe target 12. The biasing member 28 comprises a cylinder configured toreceive the sleeve 29, and a spring arranged so as to urge the sleeve incontact with the target 12. A ferrule 35 slidably holds the probesegment end 31 inside the sleeve 29. The purpose of the biasing member28 urging the sleeve 29 in contact with the target 12 is to helpcoarsely adjust the gap between the probe segment end 31 and the target12 even though the target may be vibrating.

Thermal drifting, however, may cause the sleeve 29, the probe 30, andthe probe segment end 31 to expand and contract at different rates,which leads to the gap changing. Since this is not desirable, theadjustable coupler 24 may include additional components to fine tune thegap.

For example, as shown in FIG. 3, the adjustable coupler 24′ may includea piezoelectric sleeve 32′ surrounding the probe segment end 32′, whichis in turn surrounded by the sleeve 29′. The controller 26′ applies avoltage to the piezoelectric sleeve 32′, causing the piezoelectricsleeve to expand or contract, thereby altering the length of the probesegment end 31′. This therefore allows fine tuning of the gap betweenthe probe segment end 31′ and the target 12′. The controller 26′ may becoupled to the piezoelectric sleeve 32′ via any suitable method, such assuitable electrical contacts between the sleeve 29′ and thepiezoelectric sleeve 32′.

Another embodiment of the adjustable coupler 24 is shown in FIG. 4, andincludes a temperature control unit 33″ surrounding the sleeve 29″. Thetemperature control unit 33″ is illustratively a Peltier effect unit,and is controlled by the controller 26″. The controller 26″ uses thePeltier effect unit 33″ to heat or cool the sleeve 29″ and probe segmentend 31″ to thereby cause the sleeve 29″ and probe segment end 31″ toexpand or contract, which in turn allows fine tuning of the gap betweenthe probe segment end and the target 12″.

A further embodiment of the adjustable coupler 24′″ is shown in FIG. 5,and includes a laser heating source 34′″ configured to radiate thesleeve 29′″, and thereby heat the sleeve 29′″ and probe segment end 31′″to cause the sleeve and probe segment end to expand or contract, whichin turn allows fine tuning of the gap between the probe segment end andthe target 12′″.

Referring once again to FIG. 1, in the above examples, it should beunderstood the controller 26 controls the adjustable coupler 24 basedupon an error signal. This error signal may be the DC component of thelight detected by the photodetector 20, for example.

A further embodiment of the sensing apparatus 110 is shown in FIG. 6.Here, there is no mechanically adjustable coupler, although theexcitation source 114, optical fiber 115 having an end portion 116,optical isolator 119, optical coupler 122, photodetector 120, opticalfiber 123, probe segment 130, probe segment end 131, and optical fibers115, 121 are similar to those described above with reference to FIG. 1.Rather than adjusting the gap between the probe segment end 131 and thetarget 112 such that the gap is a desired multiple of the wavelength ofthe light radiated by, and reflected into, the probe segment end, thewavelength of the interferometer laser source 118 is adjusted by thecontroller 126 such that a desired multiple of the wavelength equals thegap.

It should be understood that the sensing apparatuses 10, 10′, 10″, 10′″,110 disclosed above may include an array of excitation sources 14, 14′,14″, 14′″, 114, and an array of optical waveguide interferometers 18,18′, 18″, 18′″, 118.

With additional reference to the flowchart 40 of FIG. 7, a method ofsensing a target is now described. After the start (Block 41), waves areinduced in a target by an excitation source (Block 42). Next, theinduced waves are sensed by a fiber optic interferometer (Block 43). Thefiber optic interferometer comprises a probe segment having a probesegment end, an adjustable coupler configured to permit setting a gapbetween the probe segment end and the target.

Next, the method includes setting the gap between the probe segment endand the target using a controller coupled to the adjustable coupler(Block 44). Then, target data is generated based upon the sensed induceswaves, using the controller (Block 45). Block 46 indicates the end ofthe method.

It should be understood that the sensing apparatuses 10, 10′, 110disclosed above offer numerous advantages. For example, the use of apulsed laser 14, 14′, 114 as an excitation source allows a widebandwidth of ultrasonic waves to be induced in the target 12, 12′, 112as opposed to conventional piezoelectric excitation sources whichtypically produce more narrow bandwidths. For example, the pulsed laser14, 14′, 114 can produce ultrasonic waves with a bandwidth above 1 MHz,which is difficult to achieve with conventional piezoelectric excitationsources. In addition, with a piezoelectric excitation source, a physicalmatching layer of often required to achieve a proper acoustic impedancematch between the excitation source and the target. The sensingapparatuses 10, 10′, 110 disclosed above do not suffer this drawback andare adaptable to a wide range of target materials by adjusting theinterferometer spacing either through tuning of the interferometer laser18, 18′, 118 wavelength, or tuning of the adjustable coupler 24, 24′,124′, as opposed to using a variety of matching layers.

In addition, the ability of the sensing apparatuses 10, 10′, 10″, 10′″to either adjust the gap between the probe segment end 31, 31′, 31″,31′″ and the target 12, 12′, 12″, 12′″ or the wavelength of theinterferometer laser source 118, on the fly and based upon a feedbackerror signal provides for precise results, as effects that negativelyimpact the results can be adjusted for and mitigated. Furthermore, theuse of a pulsed laser 14, 14′, 14″, 14′″, 114 as the excitation source,coupled with the use of the fiber optic interferometer 17, 17′, 17″,17′″, 117 allows the sensing apparatus 10, 10′, 10″, 10′″, 110 to becompact and portable. Moreover, the use of optical fibers to couple thepulsed laser 14, 14′, 14″, 14′″, 114 and interferometer laser source 18,18′, 18″, 18′″, 118 to the target 12, 12′, 12″, 12′″, 112 allows thesensing of hard to reach targets, since the optical fibers may beinserted into small spaces.

The sensing apparatuses 10, 10′, 10″, 10′″, 110 disclosed herein areuseful in a wide variety of applications. For example, they may beuseful in medical imaging systems, for sensing and imaging body parts.For example, the optical fibers of the pulsed laser 14, 14′, 14″, 14′″,114 and interferometer laser source 18, 18′, 18″, 18′″, 118 may beinserted into arteries, in order to image those arteries or measure thethickness thereof, or may be inserted into a trachea in order to imagevarious components of the digestive system of a patient. Shown in FIG. 8is an embodiment where the sensing apparatus 200 (similar to the sensingapparatuses disclosed above) is a biological sensing device, and thetarget 212 is an artery having an arterial wall 250. Here, thecontroller will generate anatomical data about the arterial wall 250,such as a thickness or density of the arterial wall. Those skilled inthe art will appreciate that any biological sample or body part may besensed using this sensing apparatus 200. The illustrated referencenumbers in FIG. 8 have been increased by 200 with respect to FIGS. 1 and2 to indicate similar elements in alternative embodiments. Some of thereference numbers in FIG. 8 will not be discussed to simplify thediscussion herein, as readily understood by those skilled in the art.

In addition, the sensing apparatuses 10, 10′, 10″, 10′″, 110 may be usedfor materials inspection. For example, small welds, or welds ininaccessible places, may be inspected using the sensing apparatuses 10,10′, 10″, 10′″, 110. Wire bonds in electronic devices may be inspectedusing the sensing apparatuses 10, 10′, 10″, 10′″, 110. Hydraulic lines,such as those used in avionics systems of aircraft, or brake lines of amotor vehicle, may be inspected using the sensing apparatuses 10, 10′,10″, 10′″, 110. Shown in FIG. 9 is an embodiment where the sensingapparatus 300 (similar to the sensing apparatuses disclosed above) is amaterial inspection device, and the target 312 is a workpiece having aweld 350 to be inspected. The illustrated reference numbers in FIG. 9have been increased by 300 with respect to FIGS. 1 and 2 to indicatesimilar elements in alternative embodiments. Some of the referencenumbers in FIG. 9 will not be discussed to simplify the discussionherein, as readily understood by those skilled in the art. Here, thecontroller will generate material data about the material weld 350, suchas a thickness, density, or composition of the weld 350. Of course, thismaterial inspection device 300 need not be limited to weld inspectionand may be used to sense or inspect any sort of workpiece.

It should be understood that the specific use examples given above areby no means limiting, and that those of skill in the art will appreciatethat the sensing apparatuses 10, 10′, 10″, 10′″, 110 may be useful in anunlimited number of fields.

Referring to FIG. 10, another embodiment of a sensing apparatus 400 inaccordance with the present invention is now described. The sensingapparatus 400 is used to sense, or determine, a variety of properties ofa target 402, including, for example, the dimensions of the target, thematerial composition of the target, and the thickness of the target.

The sensing apparatus 400 includes an excitation source 404,illustratively a broadband optical source, configured to induceultrasonic waves in the target 402. The excitation source 404 is anoptical source and is illustratively coupled to the target 402 via anoptical fiber 406 having an end portion 408 in physical contact with thetarget, although it should be appreciated that in some embodiments theexcitation source is not coupled to the target via an optical fiber butrather radiates the target via free space. The excitation source 404induces the ultrasonic waves in the target 402 by rapidly heating it. Itshould be appreciated that in some applications, the excitation source404 may be a coherent optical source (e.g. a pulsed laser), or a dopedfiber amplifier. In fact, in some applications, the excitation source404 may be a pulsed laser having a spectral width that is inverselyproportional to the pulse duration.

A fiber optic interferometer 409 senses the induced waves and generatestarget data based thereupon. In particular, the fiber opticinterferometer 409 comprises a plurality of optical couplers 416, 414,422 and interconnecting optical fibers 430 a-430 e, 432 a-432 c arrangedto define a reference arm (430 a-430 e) and a measurement arm (432 a-432c). A probe segment 417 is coupled to portions of the reference arm 430c, 430 d and portions of the measurement arm 432 a. As will be discussedin greater detail below, the optical fibers 430 a-430 e making up thereference arm allow reference light as well as input light andmeasurement light to travel. The probe segment 417 has a probe segmentend 418 to be positioned adjacent the target 402 b. An optical pathlength adjustor 420 is coupled to portions of the reference arm 430 d,430 e. The optical path length adjustor 420 is illustratively apiezoelectric body, although it should be understood that any suitableoptical path length adjustor or fiber stretcher may also be used.

A reference light source 410 is coupled to the reference arm 430 a-430 eand is configured to radiate light into the reference arm, and onto thetarget 402 via the probe segment end 418. The reference light source canbe a laser source or doped fiber amplifier, as will be appreciated bythose skilled in the art. For example, the reference light source may bea high gain erbium doped fiber amplifier with a 40 nm bandwidth,centered around a wavelength of 1550 nm.

An optical power detector 412 is coupled to the reference arm 430 a-430e and is configured to receive light from the reference light source 410reflected by the target 402 into the probe segment end 418.

The plurality of optical couplers 416, 414, 422 includes a first opticalcoupler 416 coupling portions of the reference arm 430 c, 430 d toportions of the measurement arm 432 a and the probe segment 417. Asecond optical coupler 414 couples the first optical coupler 416 to thereference light source 410 and optical power detector 412. A thirdoptical coupler 422 couples portions of the reference arm 430 e toportions of the measurement arm 432 a-432 c, which thereby provides adifferential output to the photodetector 424.

A controller 426 is coupled to the optical path length adjustor 420 andis configured to adjust an optical path length of the reference arm 430a-430 e to maintain a constant relationship with respect to an opticalpath length of the measurement arm 432 a-432 c. The controller 426 mayadjust the optical path length of the reference arm 430 a-430 e basedupon the optical power detector 412 and/or the differential outputprovided to the photodetector 424.

Thermal drifting may cause the length of the optical fibers within thereference arm 430 a-430 e and the measurement arm 432 a-432 c to expandand contract at different rates, which leads to the change of theirrespective lengths. This is undesirable because it negatively affectsthe accuracy of the sensing apparatus 400. The controller 426 helpsrectify this undesirable condition by adjusting the path length of thereference arm 430 a-430 e using the optical path length adjustor 420.The matching of the path length of the reference arm 430 a-430 e and themeasurement arm 432 a-432 c by the controller 426 using the optical pathlength adjustor 420 to within 0.0025 in allows particularly accurateresults.

Operation of the fiber optic interferometer 409 is now described. Aportion of the light radiated by the reference light source 410 isradiated from the probe segment end 418 onto the target 402. This lightis then reflected from the target 402 back into the probe segment 417via the probe segment end 418, through the first optical coupler 416,through the second optical coupler 414, and into the optical powerdetector 412. The optical power detector 412 measures the optical powerreflected from the target 402, and due to the arrangement of the opticalcouplers 416, 414, 422, only the optical power reflected from thetarget. That is, the optical couplers 416, 414, 422 are arranged suchthat the light directly emitted by the reference light source 410 doesnot reach the optical power detector 412, and only the light reflectedfrom the target 402 reaches the optical power detector.

A portion of the light radiating from the reference light source 410 isconducted through the reference arm 430 a-430 e by the arrangement ofoptical couplers 416, 414, 422 and to the photodetector. Consequently,the light reflected from the target 402 and a portion of the lightradiated by the reference light source 410 and conducted through thereference arm 430 a-430 e will combine, and the superposition thereof isdetected by the photodetector 424.

The light reflected by the target 402 will typically undergo a phasechange due to the ultrasonic waves and resulting vibrations in thetarget, and therefore will have a different phase than the lightradiated by the reference light source 410 and conducted through thereference arm 430 a-430 e, causing constructive and destructiveinterference to occur therebetween. This interference therefore reflectsa detection of the sensed induced waves and can be analyzed in order todetermine various properties of the target, as will be appreciated bythose skilled in the art.

The controller 426 generates target data based upon the sensed inducedwaves. To do so, a pulse from the excitation source 404 triggers thestart of a measurement cycle, performed by the controller 426, in thetime domain. Signal peaks observed by the controller 426 correspond tothe transmit time of surface waves from the point of excitation (thatis, the point of the target 402 on which the pulse from the excitationsource 404 radiates) to the probe segment end 418. Since the distancebetween the excitation point and the probe segment end 418 is known, theacoustic velocity of the ultrasonic waves in the target 402 can becalculated. By comparing this acoustic velocity to a table of acousticvelocity for different materials, the material composition of the targetcan be determined. It should be understood that the excitation sourceprobe 408 and probe segment end 418 can be scanned to differentlocations on the target 402, so as to gather information about manypoints of the target.

The controller 426 may include a processor and a memory cooperatingtherewith. The memory may be volatile or non-volatile, and the processormay be an integrated circuit, in some applications.

In addition, the controller 426 performs typical interferometriccalculations as known to those of skill in the art on the superpositionof the light radiated from the reference light source 410 and directedthrough the reference arm 430 a-430 e and the light reflected by thetarget 402 to potentially determine the dimensions and/or the thicknessof the target.

Since a difference in the length of the path traveled by the lightreflected by the probe segment end 418 and the light radiated from thereference light source 410 and directed through the reference arm 430a-430 e will result in an additional phase difference therebetween, itis desirable for the length of the reference arm 430 a-430 e and thelength of the measurement arm 432 a-432 c to remain the same, or atleast for a constant relationship between the length of the referencearm and measurement arm to be maintained. If a constant relationshipbetween the length of the reference arm 430 a-430 e and the measurementarm 432 a-432 c is to be maintained, it is desirable for the differencein length to be a desired multiple of the wavelength of the lightradiated by reference light source 410. The multiple used need not be aninteger in some embodiments, and need not be greater than one in someembodiments.

It should be appreciated that the optical path length adjustor 420 neednot operate by physically changing a length of an optical fiber in allembodiments. For example, the optical path length adjustor 420 may be anadjustable delay line or phase modulator which can maintain a constantphase relationship between the light reflected from the target 402 andthe light radiated by the reference light source 410 and through thereference arm 430 a-430 e. The maintenance of a constant phaserelationship between the light in the reference arm 430 a-430 e and themeasurement arm 432 a-432 c also helps to provide accurate results.

In some applications, the reference arm 430 a-430 e may even include afree space element. One such embodiment is now described with referenceto FIG. 11. The illustrated reference numbers in FIG. 11 have beenincreased by 100 with respect to FIG. 10 to indicate similar elements inalternative embodiments. Some of the reference numbers in FIG. 11 willnot be discussed to simplify the discussion herein, as readilyunderstood by those skilled in the art. Here, the sensing apparatus 500remains the same as the sensing apparatus 400 of FIG. 10, except thatthe reference arm 530 a-530 e includes a free space element. Here, thefree space element is contained within an adjustable lens arrangement521. The reference optical fiber 530 d terminates at a coupler on thefirst side of the adjustable lens arrangement 521, and radiatesreference light via free space and through a first lens 523. Thereference light then passed through a second lens 525, which focuses thelight back into the reference optical fiber 530 e via another coupler.The distance between the first lens 523 and second lens 525 isadjustable based upon input received from the controller 526. Thisthereby allows adjustment of the length of the path of the reference arm530 a-530 e.

A method of operating a sensing apparatus is now described withreference to the flowchart 550 of FIG. 12. The sensing apparatusincludes a fiber optic interferometer comprising a plurality of opticalcouplers and interconnecting optical fibers arranged to define areference arm, a measurement arm, a probe segment coupled to thereference arm and the measurement arm and having a probe segment end,and an optical path length adjustor coupled to the reference arm.

After the start of the method (Block 551), the waves are induced in atarget using an excitation source (Block 552). Then, a probe segment endis positioned adjacent the target (Block 553).

An optical path length of the reference arm is then adjusted via theoptical path length adjustor to maintain a constant relationship withrespect to an optical path length of the measurement arm, using acontroller (Block 554). The induced waves are then sensed using aphotodetector coupled to the controller (Block 555). Target data is thengenerated based upon the sensed induced waves, using the controller(Block 556). Block 557 indicates the end of the method.

It should be understood that the sensing apparatuses 400, 500 disclosedabove may include an array of excitation sources 404, 504, and an arrayof fiber optic interferometers 409, 509.

The sensing apparatuses 400, 500 disclosed herein are useful in a widevariety of applications. For example, they may be useful in medicalimaging systems, for sensing and imaging body parts. For example, theoptical fibers 406, 408, 506, 508 of the excitation source and referencelight source 410, 510 may be inserted into arteries, in order to imagethose arteries or measure the thickness thereof, or may be inserted intoa trachea in order to image various components of the digestive systemof a patient. Shown in FIG. 13 is an embodiment where the sensingapparatus 600 includes optical fibers 608 and 618 (similar to thesensing apparatuses 400 and 500 disclosed above) is a biological sensingdevice, and the target is an artery having an arterial wall 650. Here,the controller will generate anatomical data about the arterial wall650, such as a thickness or density of the arterial wall. Those skilledin the art will appreciate that any biological sample or body part maybe sensed using this sensing apparatus 600.

In addition, the sensing apparatuses 400, 500 may be used for materialsinspection. For example, small welds, or welds in inaccessible places,may be inspected using the sensing apparatuses 400, 500. Wire bonds inelectronic devices may be inspected using the sensing apparatuses 400,500. Hydraulic lines, such as those used in avionics systems ofaircraft, or brake lines of a motor vehicle, may be inspected using thesensing apparatuses 400, 500. Shown in FIG. 14 is an embodiment wherethe sensing apparatus 700 includes optical fibers 708 and 718 (similarto the sensing apparatuses disclosed above) is a material inspectiondevice, and the target 702 is a workpiece having a weld 750 to beinspected. Here, the controller will generate material data about thematerial weld 750, such as a thickness, density, or composition of theweld 750. Of course, this material inspection device 700 need not belimited to weld inspection and may be used to sense or inspect any sortof workpiece.

Other details of such sensing apparatuses 10 may be found in co-pendingapplications INTERFEROMETRIC BIOMETRIC SENSING APPARATUS INCLUDINGADJUSTABLE COUPLING AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,654filed May 6, 2011, now U.S. Patent Application Publication No.2012/0281228 published Nov. 8, 2012; INTERFEROMETRIC MATERIAL SENSINGAPPARATUS INCLUDING ADJUSTABLE COUPLING AND ASSOCIATED METHODS, U.S.Ser. No. 13/102,686 filed May 6, 2011, now U.S. Patent ApplicationPublication No. 2012/0281229 published Nov. 8, 2012; INTERFEROMETRICSENSING APPARATUS INCLUDING ADJUSTABLE REFERENCE ARM AND ASSOCIATEDMETHODS, U.S. Ser. No. 13/102,712 filed May 6, 2011, now U.S. PatentApplication Publication No. 2012/0281230 published Nov. 8, 2012;INTERFEROMETRIC BIOLOGICAL SENSING APPARATUS INCLUDING ADJUSTABLEREFERENCE ARM AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,732 filed May6, 2011, now U.S. Patent Application Publication No. 2012/0281231published Nov. 8, 2012; and INTERFEROMETRIC MATERIAL SENSING APPARATUSINCLUDING ADJUSTABLE REFERENCE ARM AND ASSOCIATED METHODS, U.S. Ser. No.13/102,755 filed May 6, 2011, now U.S. Patent Application PublicationNo. 2012/0281232 published Nov. 8, 2012, the entire disclosures of whichare hereby incorporated by reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A sensing apparatus comprising: anexcitation source configured to induce waves in a target; and a fiberoptic interferometer configured to sense the induced waves in the targetand comprising a laser source, a probe segment having a probe segmentend to be positioned adjacent the target and defining a gaptherebetween, an optical coupler operatively connecting said lasersource and said probe segment, and a controller coupled to said lasersource and configured to adjust a wavelength of said laser source sothat a multiple of the wavelength equals the gap between the probesegment end and the target.
 2. The sensing apparatus of claim 1, whereinsaid fiber optic interferometer further comprises a photodetectorcoupled to said controller; and wherein said optical coupler alsooperatively connects said photodetector to said probe segment and saidcontroller.
 3. The sensing apparatus of claim 1, wherein said controlleris further configured to generate target data based upon the sensedinduced waves.
 4. The sensing apparatus of claim 1, wherein said probeend comprises an optical fiber with an angled endface.
 5. The sensingapparatus of claim 1, wherein said excitation source comprises at leastone pulsed laser.
 6. A sensing apparatus comprising: an excitationsource configured to induce waves in a target; and a fiber opticinterferometer configured to sense the induced waves in the target andcomprising a laser source, a probe segment having a probe segment endcomprising an optical fiber with an angled endface to be positionedadjacent the target and defining a gap therebetween, an optical coupleroperatively connecting said laser source and said probe segment, and acontroller coupled to said laser source and configured to adjust awavelength of said laser source so that a multiple of the wavelengthequals the gap between the probe segment end and the target; saidcontroller also configured to generate target data based upon the sensedinduced waves.
 7. The sensing apparatus of claim 6, wherein said fiberoptic interferometer further comprises a photodetector coupled to saidcontroller; and wherein said optical coupler also operatively connectssaid photodetector to said probe segment and said controller.
 8. Thesensing apparatus of claim 6, wherein said excitation source comprisesat least one pulsed laser.
 9. A method of sensing a target using a fiberoptic interferometer comprising a laser source, a probe segment having aprobe segment end to be positioned adjacent the target and defining agap therebetween, and an optical coupler operatively connecting saidlaser source and said probe segment, the method comprising: inducingwaves in the target using an excitation source; sensing the inducedwaves in the target using the fiber optic interferometer; and adjustinga wavelength of said laser source so that a multiple of the wavelengthequals the gap between the probe segment end and the target using acontroller.
 10. The method of claim 9, wherein the fiber opticinterferometer further comprises a photodetector coupled to thecontroller; and wherein the optical coupler also operatively connectsthe photodetector to the probe segment and the controller.
 11. Themethod of claim 9, further comprising generating target data based uponthe sensed induced waves using the controller.
 12. The method of claim9, wherein the probe end comprises an optical fiber with an angledendface.
 13. The method of claim 9, wherein the excitation sourcecomprises at least one pulsed laser.